The expression of ‘Hairless’ inDrosophilaand the role of two closely linked modifiers of opposite effect

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Abstract
1.Hairless(H), a dominant mutant ofDrosophila melanogaster, affects the chaetae.2. Various ‘intensities’ of effect are interpreted as representing different times in chaetal development at which deficiency of ‘+Hsubstance’ is encountered. ‘Temperature effective period’ studies confirm this view.3. The chaetae at different sites are liable to characteristically different ‘intensities’ of effect. In other words a particular site tends to be affected at a limited time in its development. If it is supposed that a single period of deficiency of +Hsubstance accounts for effects on all chaetae, then it follows that development of the various chaetae is asynchronous. Some histological observations tend to support this prediction.The implications of asynchrony with respect to pattern formation are discussed.4. It is argued that the source of +Hsubstance may be an endocrine organ, the ring-gland.5. Two modifiersSu-HandE-H, whose expression and linkage relations are described in detail for the first time, are, it is suggested, respectively ‘constitutive’ and ‘super-repressed’ mutations of a ‘regulator’ locus controlling negative feed-back on the +Hlocus.